This invention relates to a master cylinder comprising a cylinder body, and a piston slidably inserted in the bore of the cylinder body so as to be kept in slide contact with a cup fitted in the inner wall of the bore to seal the gap between the outer surface of the piston and the inner wall of the bore.
The cup is sandwiched between piston guide portions formed on the inner wall of the cylinder body to guide the piston in slide contact with the piston. The piston guide portions are formed with grooves through which brake fluid is supplied from a reservoir into a pressure chamber defined in the cylinder body. By forming the grooves, brake fluid can be supplied from behind the cup, which is typically fitted in an annular groove formed in the inner wall of the bore of the cylinder body, to the front of the cup through these grooves.
Such grooves are disclosed in French patent 2820387 and U.S. Pat. No. 4,524,585. The grooves disclosed in the French patent are straight and helical grooves. The US patent discloses an even number of straight grooves that are arranged at equal circumferential intervals.
Specifically, the master cylinder of the French patent includes a cylinder body having piston guide portions formed on its radially inner surface and having an inner diameter slightly larger than the outer diameter of the piston so as to guide the piston while being in slide contact with the piston. The grooves are formed in the radially inner surface of each piston guide portion. The grooves have bottoms that lie on an imaginary cylinder having a diameter greater than the opening of the bore of the cylinder body. Such grooves can be formed only by machining and not by any other method. Thus, the productivity of the entire master cylinder is low especially where the grooves are helical grooves.
The grooves disclosed in the US patent have also bottoms that lie on an imaginary cylinder having a greater diameter than the opening of the bore of the cylinder body. The productivity of the entire master cylinder is thus low for the same reason as mentioned above.
An object of the present invention is to provide a master cylinder of the abovementioned type having grooves which can be formed easily and which are less likely to scratch or otherwise damage the piston even if the piston is rubbed against the edges of the grooves.